Carbon nanocoils that are wound in coil form of which the outer diameter is no greater than 1000 nm have been synthesized. Carbon nanocoils have similar properties as carbon nanotubes and significant electromagnetic induction, and thus, are useful for the material for the head of a hard disc and for an absorber of electromagnetic waves. In addition, carbon nanocoils have spring elasticity that makes it return to its original length when expanded to a length that is twice as long, and therefore, have been attracting attention as a material for the springs of micro-machines and actuators, as well as a resin reinforcing material.
Carbon nanocoils were synthesized for the first time using a chemical vapor deposition method (hereinafter referred to as CVD method), by Amelinckx et al. (Amelinckx, X. B. Zhang, D. Bernaerts, X. F. Zhang, V. Ivanov and J. B. Nagy, SCIENCE, 265 (1994) 635) in 1994. It was also clarified that carbon micro-coils that had been synthesized before had an amorphous structure, whereas carbon nanocoils had a graphite structure.
In accordance with their synthesizing method, a single metal catalyst, such as Co, Fe and Ni, is formed as microscopic powder, and the vicinity of this catalyst is heated to 600° C. to 700° C., and then, an organic gas, such as acetylene or benzene, is made to flow so as to make contact with this catalyst, so that these organic molecules are decomposed. However, the form of the generated carbon nanocoils varies, and the yield is low, indicating that carbon nanocoils were merely accidentally generated. That is, the method cannot be industrially utilized, and a more efficient synthesizing method has been in demand.
Li et al. (W. Li, S. Xie, W. Liu, R. Zhao, Y. Zhang, W. Zhou and G. Wang, J. Material Sci., 34 (1999) 2745) succeeded in generating carbon nanocoils using a new method in 1999. In accordance with their synthesizing method, a catalyst where the outer periphery of a graphite sheet is coated with iron particles is placed in the middle, and the vicinity of this catalyst is heated to 700° C. using a nichrome wire, and then, a mixed gas of 10% by volume of acetylene and 90% by volume of a nitrogen gas is brought into contact with this catalyst, so as to cause a reaction. However, this synthesizing method also has a small yield of coils, and is insufficient as an industrial method for mass production.
The key for increasing the yield of carbon nanocoils in accordance with a CVD method is in the development of an appropriate catalyst. Considering this point, some of the present inventors developed an Fe.In.Sn-based catalyst so as to obtain a yield of no less than 90%, and this achievement was disclosed as Japanese Patent Laying-Open No. 2001-192204 (Patent Document 1). This catalyst is formed by vapor depositing an iron thin film on an ITO substrate on which a mixed thin film of an In oxide and an Sn oxide has been formed. ITO is an abbreviation of indium-tin-oxide.
In addition, some of the present inventors formed an Fe.In.Sn-based catalyst in accordance with another method and succeeded in synthesizing a large amount of carbon nanocoils, and this achievement was disclosed as Japanese Patent Laying-Open No. 2001-310130 (Patent Document 2). This catalyst is formed by mixing an In organic compound and an Sn organic compound into an organic solvent so as to form an organic liquid, applying this organic liquid to a substrate so as to form an organic film, baking this organic film so as to form an In.Sn oxide film, and forming an iron thin film on this In.Sn oxide film. The In.Sn oxide film corresponds to the aforementioned ITO film (mixed thin film).
Meanwhile, research aiming to increase the efficiency of the catalyst by making a particular carrier carry a compound catalyst has been conducted. Research in this field was conducted in the area of carbon nanotubes and disclosed in Japanese Patent Laying-Open No. 2002-255519 (Patent Document 3) and Japanese Patent Laying-Open No. 2003-313017 (Patent Document 4).
These Patent Documents 3 and 4 relate to a synthesizing method of single layer carbon nanotubes. Both of these known technologies relate to a technology for generating carbon nanotubes by making zeolite absorb a catalyst for synthesizing carbon nanotubes. It has been reported that the generated carbon nanotubes have a relatively uniform tube diameter. That is, the technology has an object of synthesizing relatively uniform carbon nanotubes having the diameter of the microscopic pores by making the microscopic pores of the zeolite absorb the catalyst.
Patent Document 1: Japanese Patent Laying-Open No. 2001-192204
Patent Document 2: Japanese Patent Laying-Open No. 2001-310130
Patent Document 3: Japanese Patent Laying-Open No. 2002-255519
Patent Document 4: Japanese Patent Laying-Open No. 2003-313017
The present inventors began recognizing an interesting fact while energetically conducting research for synthesizing carbon nanocoils in accordance with a CVD method using the Fe.In.Sn-based catalyst that were developed in Patent Documents 1 and 2. This is the fact that a substance in particle form attaches itself to the ends of carbon nanocoils as seen in photomicrographs. The present inventors refer to this substance in particle form as catalyst nuclei.
The present inventors came to think that the catalyst nuclei that attach themselves to the ends of carbon nanocoils are a true catalyst substance. That is, these catalyst nuclei decompose carbon compound gas that exists around the catalyst nuclei so as to make carbon nanocoils grow while taking in carbon atoms. Carbon nanocoils are a microscopic carbon substance, and therefore, the substance in catalyst form that attaches itself to the ends of carbon nanocoils is ultra-fine particles of a nano-size.
It is an extremely difficult task to sample one carbon nanocoil so as to directly analyze one microscopic catalyst nucleus that attaches itself to the end of this carbon nanocoil. The catalyst nucleus is extremely small and easily falls off, and therefore, it is extremely difficult to determine its composition formula or structure in accordance with a physical or chemical technique. In addition, it is also a difficult task to take a high resolution transmission electron microscope image of such a catalyst nucleus.
However, in case these catalyst nuclei are a true catalyst, it is extremely important to determine their structure. That is, it became an extremely important issue for the present inventors to determine whether these catalyst nuclei were mere microscopic pieces of the Fe.In.Sn-based catalyst or made of another substance. This is because there was a possibility that a more effective catalyst for synthesizing carbon nanocoils could be provided by determining the structure of these catalyst nuclei.
In addition, in accordance with the known technology disclosed in Patent Document 3, zeolite is made to absorb fine particles of Fe and fine particles of Ni as fine particles of a catalyst. The fine particles of Fe and the fine particles of Ni are much greater in size than the molecules of compounds that dissolve, and therefore, there is a drawback that fine particles of the catalyst cannot be absorbed by the pores of the zeolite in the case where the diameter of the pores is small. In addition, even in the case where the fine particles of the catalyst are absorbed by the pores, the diameter of pores has a distribution in a certain range, and there is dispersion in the tube diameter, in accordance with this distribution. Furthermore, the diameter of fine particles of a metal that mono-disperses is approximately 10 nm in the state of the art. When no greater than 10 nm, the fine particles of the metal combine with each other so as to lump together, and the diameter of the secondary particles that have lumped together reaches several tens of nm or higher, and therefore, there is a drawback that carbon nanotubes having an extremely large tube diameter grow when attached to the surface of this zeolite.
Patent Document 4 discloses a technology for making zeolite absorb iron nitrate molecules in a solution. That is, it has been found that iron nitrate molecules are absorbed by the pores of zeolite, and the ratio of filling of the catalyst into the pores becomes higher than that of the aforementioned fine particles of a catalyst. However, catalysts that are absorbed by zeolite are one type of metal element or a substance that contains such a metal, and not a mixture of a plurality of types of metal elements or a substance that contains such metals. Catalysts of carbon nanotubes are fine particles of single Fe or fine particles of single Ni, and therefore, are possible to uniformly inject into the pores of zeolite. However, catalysts for synthesizing carbon nanocoils are formed of a plurality of types of metals, like the Fe.In.Sn catalyst, and therefore, it is necessary to simultaneously fill the same pore with a plurality of types of metal. It is easy to understand that it is difficult to simultaneously inject a plurality of types of metals into the same pore. Accordingly, it is totally unknown whether or not a plurality of catalysts for synthesizing carbon nanocoils can be absorbed by zeolite, and such an experiment has never been carried out.
Accordingly, an object of the present invention is to identify the true catalyst for synthesizing carbon nanocoils, by indirectly determining the structure of the catalyst nuclei attached to the ends of carbon nanocoils, establish a synthesizing method of such a catalyst, and synthesize carbon nanocoils of a high density with high efficiency in a short period of time. Another object is to develop a novel catalyst for synthesizing carbon nanocoils, other than the Fe.In.Sn catalyst. Still another object is to provide a novel catalyst for synthesizing carbon nanocoils, where this novel catalyst substance is carried by a porous carrier. Yet another object is to establish a method for synthesizing carbon nanocoils using this novel catalyst for synthesizing carbon nanocoils, and provide uniform and inexpensive carbon nanocoils for the market.